Process for forming wrappers of thin sheet materials and a device for carrying out same

ABSTRACT

This invention pertains to a method and apparatus for wrapping an article, wherein a metal foil is pleated and then formed in to a dished shape such that the pleats unfold, and then an article is placed within this dished shape.

The present invention relates in general to the formation of wrappers(or coverings) of thin sheet materials and has been developed withparticular attention to its possible use for producing wrappers forconfectionery products such as, for example, chocolate eggs.

In this field of application, reference to which should not, however, beinterpreted in a limiting sense, there is generally a problem in fittingthe shape of the wrapper (which typically is made of thin sheet materialsuch as, for example, aluminium foil) to the shape of the product to bewrapped. The product may include one or more rounded portions or may bewholly constituted by such portions; for example, it may be a chocolateegg constituted by two complementary portions each of which has arounded shape; the same can also be said of generally spherical productsand the like.

Up to now, two different solutions have been used to form wrappers ofthin sheet material for such products.

The first solution, the use of which predominates widely in industry, isthat of taking the generally flat sheet material and wrapping it aroundthe product to be wrapped by means of automatic or semi-automaticequipment which copies the sequence of steps which would be carried outby a human operator.

This solution is certainly satisfactory for many applications,particularly when the wrapping sheet is uniform in appearance (forexample, a sheet of silvered or gilded aluminium) so that the random orpseudo-random distribution of folds formed in the sheet when it iswrapped around the product has no particular relevance; the distributionof folds may thus actually help to make the appearance of the wrappedproduct more pleasing, at least in some cases. If pictures, drawings orwording are applied to the wrapper, however, the situation is moredifficult; in this case, the formation of folds in the wrapping sheetmay detract from a general appreciation of the graphic symbols and, atleast in some cases, may make the wording applied to the wrapperpractically illegible.

Another solution which is documented, for example, by Italian patent651,202, German patent 598,113, and also German patent 1,784,647provides for the sheet wrapper to be preformed before it is applied tothe product to be wrapped so that it assumes beforehand a shape whichexactly fits the product to be wrapped. The preforming of the sheetmaterial can be controlled precisely so as to limit, if not completelyeliminate, the formation of folds, thus preventing the problems citedabove. This solution has found considerable success, particularly withplastics wrapping materials, for which the preliminary forming operationcan be effected by heating the material so as to soften it and thenforming it by various moulding techniques.

The application of this solution to metal wrapping sheets, typicallyaluminium foil, however, appears more difficult. In practice, it isfound that the technique of preforming metal foil can be usedsuccessfully only with materials of a certain thickness (for example, afew tenths of a millimeter, as is the case with some aluminium foilwrappers used for some pharmaceutical products, such as suppositories,or for making trays for holding products).

Furthermore, U.S. Pat. No. 3,748,889 disclosed a method and a dieassembly for pleating a thin sheet of material, which permits theworkpiece to be gathered and stretched by means of a progressivedeformation which minimises the risk of tearing and splitting thematerial.

Without wishing to be bound to any particular theory in this connection,one has reason to believe that the chances of success with thistechnique are linked essentially to the use of materials of thicknessessuch that the wrapper or covering can be preformed by a drawing process,that is, with a certain stretching of the material, without causingtears.

When the material of the wrapper is thin, however, (as in the case ofaluminium foil of the order of 10-20 μ thick) it is practicallyimpossible to preform the foil (for example, to give it a dished shapelike the portions which constitute a chocolate egg) without it tearingor splitting.

According to a solution tested by the Applicant, the tearing of thewrapper or covering during the preforming can be prevented, to a certainextent, by subjecting the foil to an embossing process, which term, inthe field of sheet materials, means the formation of a dense pattern ofsurface irregularities (for example, in the form of small cones,cylinders, or a generally "ravioli-like" structure) with dimensionsgenerally smaller than 1 mm. The Applicant has found, however, that sucha solution is unsatisfactory since it does not bring the tearing of thesheet material during preshaping below a statistically appreciablelevel.

The object of the present invention, therefore, is to provide a methodof forming wrappers of thin sheet material, particularly metal foil,which does not give rise to the problems described above.

According to the present invention, this object is achieved by virtue ofa method having the specific characteristics recited in claim 1.Advantageous developments of the invention are recited in claims 2 to26.

Another subject of the invention is a device or tool for carrying outthe method of the invention as claimed in claim 27. Further advantageousdevelopments of this device are recited in claims 28 to 39.

Briefly, the present invention is based on the recognition of the factthat a wrapper of a thin material, particularly metal such as, forexample, aluminium, can be formed into a generally dished shape, forexample, in order to form part of a wrapper for covering one of thehalves of a chocolate egg, without the risk of tearing, provided that,before the forming step, the sheet undergoes a pleating process which ispreferably effected along meridians of the dished shape (in the mannerwhich will be explained further below).

By way of premise, it should be noted that, the term "pleating" as usedin the present description and the in the following claims, refers tothe formation, in a sheet material, of a series of folds or pleatshaving dimensions generally larger than one millimeter, typically of theorder of at least a few millimeters, for example, 5-8 millimeters ormore.

The invention will now be described, purely by way of non-limitingexample, with reference to the appended drawings, in which:

FIGS. 1 to 8 show schematically various steps of a method according tothe invention,

FIGS. 9 and 10 show in greater detail the results obtained by some ofthe steps of FIGS. 1 to 8,

FIG. 11 shows in greater detail the structure of a device for use in themethod according to the invention,

FIG. 12 is a section taken on the line XII--XII of FIG. 11, and

FIGS. 13 to 15 show the operation of the device of FIGS. 11 and 12, ingreater detail.

The present invention will be described, by way of example, withreference to the formation of a thin aluminium wrapper (with a thicknessof the order of 10-20 μ, typically 11.4 μ) for a product constituted bya chocolate egg U of substantially the same dimensions as a hen's egg.

In particular, FIGS. 1 to 4 show schematically the sequence of stepswhich lead to the formation of one of the two symmetrical halves of thewrapper, that is, of the dished portion (shown in detail in FIG. 10)which is intended to cover one half of the egg U. FIGS. 5 to 8, however,show how two dished portions formed by the sequence of steps of FIGS. 1to 4 can be arranged around a chocolate egg U so as to form a wrapper ofthin sheet material.

As already indicated, the method of the invention may, however, be usedfor different applications, and hence for wrapping products other thanconfectionery or food products in general or for wrapping products whichare not egg-shaped or spherical, for example, for wrapping products suchas roughly hemispherical pralines. In the latter case, the method of theinvention could, to advantage, be used to form the dished portion forcovering the hemispherical portion of the product. The dished portionformed according to the invention could be joined, for example, to aflat or cup-shaped sheet constituting the portion of the wrapper whichis wrapped around the base of the product. The method of the inventionmay also, to advantage, be used to form only a particular dished portionof part of a larger wrapper.

FIG. 1 shows how the sheet wrapping material F (for example, aluminiumfoil 11.4 μ thick) can be passed through a unit including a punch 1 anda die 2 having the specific characteristics recited below with referenceto FIGS. 11 and 12, in order to form a generally pleated effect therein.

In the industrial method, the sheet wrapping material F advances towardsthe punch-die unit 1, 2 from an unwinding reel (not shown) and may bedivided, beforehand or simultaneously, into small sheets for formingrespective dished wrapping portions.

As will become clearer from the following, the die 2 (or the femaleportion of the pleating device) represents a general development in aplane of the overall shape of the product to be wrapped by the sheet F.For example, in the case of a spherical product, the die 2 will begenerally circular, whereas in the case of an egg-shaped product (whichis the specific case to which the present description relates) it willbe generally oval (and flat). In both cases, the die 2 has a series ofgrooves 3 of approximately triangular cross-section extending generallyalong meridians of its shape. For example, in the case of an oval die(see also FIG. 11), the grooves 3 extend in a generally symmetricalarrangement from the base of the egg-shape towards its tip. As alreadystated, the grooves 3--and hence the corresponding pleats formed in thesheet F--have dimensions of the order of at least a few millimeters.

FIG. 2 shows schematically the situation in which the punch 1 and thedie 2 have come into contact with each other, clamping the sheet Fbetween them and thus forming the pleats therein.

As will become clearer from the following, during the pleatingoperation, the sheet F is not clamped completely rigidly but retains acertain ability to slide between the punch 1 and the die 2, giving riseto a general inward movement towards the centre of the pleating device1, 2 as the pleating operation gradually proceeds. This movement towardsthe centre is facilitated, as will become clearer from the following, bythe conformation of the punch 1 which, in the rest condition, has agenerally three-dimensional shape with central portions which projectfurther than its peripheral portions, and which--during its movement inorder to mate with the die 2--gradually changes to a generally flatconfiguration.

The Applicant has found that the ability to slide, which is alsofacilitated by the presence of a lubricant lacquer (for example, apolyethylene lacquer) on at least one or, even better, on both faces ofthe sheet F, is extremely advantageous for preventing the risk oftearing.

The sheet F which is being pleated (shown in detail in FIG. 9) is thenpassed (FIGS. 3 and 4) to a further forming device which is alsoconstituted by a punch 4 and a die 5. These are intended to effect theactual forming of the already-pleated sheet F, giving it a shapeprecisely corresponding to that of the product (the egg U in theembodiment to which the present description relates) to be covered bythe sheet.

In the specific embodiment, the punch 4 and the die 5 will thereforehave positively and negatively dished (half) egg-shapes, respectively.

As a result of the mating of the punch 4 and the die 5 (see FIG. 4), thesheet F assumes the dished, egg-shaped configuration to be impartedthereto, as shown in FIG. 10.

As a result of the previous pleating process (FIGS. 1 and 2), thedishing can be achieved without risk of tearing, even with very thinsheets F, for example, 11.4 μ thick aluminium foil.

Without wishing to be bound to any particular theory in this connection,one has reason to believe that the main effect of the pleating operationis to prevent any stretching of the material (which does, however, occurin solutions in which the forming is carried out directly on a flatsheet) during the subsequent forming operation, even as a result of itsslight movement towards the centre, mentioned above.

In this case, it has also been found that the presence of a lacquerwhich has a lubricating effect on one or, even better, on both faces ofthe sheet F is beneficial in further reducing any risk of tearing.

Moreover, it has been found advantageous for both the punch 4 and thedie 5 to have ducts 6, 7 for quickly discharging from the device anymasses of air trapped between the punch 4 and the sheet F, on the onehand, or between the sheet F and the die 5, on the other hand,particularly when--as occurs in industrial processes--the pressing is tobe carried out particularly quickly (with pressing times of less thanone second).

At this point, the two half-egg-shaped wrapper pressings produced fromthe sheet F as shown in FIG. 10 can be used to cover a chocolate egg U.

For this purpose, a first half-egg-shaped pressing F can be placed in aholding die 8 (which may in fact correspond to the die 5 used for theforming) in order to receive the egg U (which is formed according towidely known criteria in a moulding line, not shown) so that only theupper half of the egg U is left projecting from the die 8.

Another pressing such as that shown in FIG. 10 can then be positionedthereon, so that the egg U is completely surrounded by sheet wrappingmaterial.

At this point, it is necessary to close the formed wrapper and this canbe achieved by joining the two half-egg-shaped wrapper pressings Ftogether along their mutually facing portions around the periphery ofthe egg U enclosed between them.

This can be achieved, at least in principle, by various techniques, suchas mechanical joining by folding, gluing, ultrasonic welding,heat-sealing of the material constituting the wrapper pressings F, etc.,possibly with the simultaneous cutting of the sheets F which are joinedtogether around the periphery of the egg U.

In this connection, the need to satisfy a set of concomitantrequirements should, however, be noted.

In the first place, the two aluminium wrapper pressings F must be joinedwithout damaging the egg U, even locally. This risk may arise if the twosheet wrapper pressings F are joined together by heat-sealing thematerial of which they are made.

On the other hand, it is desirable for the sheet pressings F to be cutas close as possible to the product U in order to prevent the rim or"Saturn's ring" 11 which is formed around the egg U as a result of thecutting of the aluminium pressing from projecting too far, which wouldadversely affect its appearance even if the rim or ring 11 were then tobe folded against the product U according to a current solution.

In this connection, the solution (which has already been mentioned forother reasons) of using a sheet material F which has a layer ofheat-sealable lacquer or resin, for example polyethylene lacquer, on atleast one of its faces (and, in particular, on the face which isintended to face the product U) has been found particularlyadvantageous.

As has been seen, the presence of the layer of lacquer is beneficial foreffecting the pleating operation. Moreover, the lacquer is whollycompatible with use for wrapping food products. Furthermore, the lacquerenables the two wrapper pressings F to be welded together easily by thelowering of a so-called heating muffle 9 near the region of the jointbetween the two pressings F around the periphery of the product U. Inpractice, the muffle 9 is constituted by an annular heated body definingan aperture, the shape of which closely imitates the outline of the eggU between the two pressings F of sheet material. Once it bears on thelips of the two pressings which are in contact with each other, as shownschematically in FIG. 6, the muffle 9, which is brought to a temperatureof the order of 80°, can melt the lacquer or resin locally so as to weldand securely join the two pressings F of sheet material together withina period of about 1 second. This causes no damage to the egg U (bothbecause of the relatively low temperature and because of the short timeof application).

The joint formed between the two pressings F of sheet material by thelocal melting of a lacquer or resin which covers their mutually facingsurfaces is also beneficial for ensuring that the wrapper is completelysealed, even if there are slight folds or wrinkles along the mouthportions of the two pressings which are joined together.

The welding together of the two pressings F of sheet material, to whichFIG. 6 relates, is followed by the cutting of the wrapper around theperiphery of the egg U. This can be achieved with the use, for example,of a hollow punch 10 with a cutting edge 10a, the profile of whichcorresponds approximately to the outline of the product U in the regionin which the two pressings F of sheet material are joined. In general,the cutting edge 10a follows the outline of the product U with a certainclearance so that its cutting action forms the rim or outer ring 11constituted by the joined portions of the sheet material F when the tool10 is removed (also removing the flat outer peripheral portions of thetwo pressings F of sheet material). This ring or rim forms a sort offlange 1 or 2 mm wide which projects from the finally wrapped product U.The ring or rim 11 can be folded against the outer surface of thewrapper by a subsequent folding operation.

In other applications, however, the ring or rim 11 may be left unchangedsince its presence does not adversely affect the appearance of the finalproduct.

Continuing with a more detailed examination of the structure of thepleating device 1, 2, (see in particular FIGS. 11 and 12), it can beseen that at least one and, preferably, both of the punch 1 and the die2 have restraining formations 12, 13 in the generally flat regionssurrounding the actual male and female die portions (which have thecharacteristics described in greater detail below), the restrainingformations 12, 13 being constituted, for example, by portions 12 offoamed strip stuck to the flat surface of the male die 1 and an almostcomplete covering 13 on the flat face of the female die (the matrix) 2,also formed, for example, by an adhesive foamed strip.

Naturally, the relative positions of the portions 12 and of the covering13 could be reversed with the portions 12 on the female die 2 and thesubstantially continuous covering 13 on the flat face of the male die 1.

As stated, the restraining elements 12 are formed, for example, byfoamed strips each having an adhesive surface so that it can be appliedfirmly to the flat face of the respective die element 1 or 2, leavingfree a generally smooth surface (that is, the surface facing thecomplementary die) which has a certain ability to yield resiliently.

The formations 12 and 13 are intended to grip the sheet F when it isinterposed between the die portions 1 and 2 for the pleating operation.

The formations 12 and 13 actually grip the sheet F in a generallyyielding or gentle manner, in the sense that they do not completelyoppose the gradual movement of the sheet F towards the centre of thepleating device.

It can be seen in the lower portion of FIG. 11, which relates to thematrix or female die 2, that the shaped portion of the die 2 has a setof ribs or grooves 3 which are oriented along its meridians for formingthe pleats. By way of example, with reference to the formation of awrapper for wrapping a chocolate egg having the size of a normal hen'segg, the matrix 3 may have a set of five grooves with generallytriangular or V-shaped cross-sections, including a central groove, twointermediate side grooves and two outer side grooves. Naturally,different solutions may be used, according to the dimensions and thegeneral shape of the product to be covered. The same configuration ofshaped portions is naturally used in a complementary manner, that is, inthe form of ribs, on the male die or actual punch 1, which is shown ingreater detail in the upper portion of FIG. 11 and, in section, in FIG.12.

In the embodiment illustrated, the male die 1 thus also has five ribswith triangular or V-shaped cross-sections.

More precisely, it has a central rib 14, two intermediate side ribs 15and two outer side ribs 16 for cooperating respectively (with theinterposition of the sheet F to be pleated) with the central groove, theintermediate side grooves and the outer side grooves of the female die2.

As already stated in the introduction to the present detaileddescription, whereas the female die or matrix 2 is generally flat (thisterm meaning it has a discontinuous profile extending substantially inalignment with the flat portion of the die 2), the male die 1, however,has a profile which--in the rest condition (that is, before it engagesthe die 2)--is not flat, in the sense that the central rib 14 projectsbeyond the intermediate side ribs 15 which in turn project beyond theouter side ribs 16. Naturally the term "project beyond" is intended todefine a situation in which the respective rib projects or extendsfurther from the die 1 than the adjacent outer ribs.

This can be achieved, as shown schematically in FIG. 12, if the outerside ribs 16 are formed as parts which are integral with or fixed to thestructure of the die 1 but the intermediate side ribs 15 and the centralrib 14 are formed as blocks or punches which can slide relative to thebody of the die 1.

Specifically, the inner boundaries of the outer side ribs 16 of thepunch 1 define a generally oval or lenticular cavity 17 in which theintermediate side ribs 15 (which are thus generally C-shaped) can slide.The intermediate ribs 15 in turn define a further lenticular cavity inwhich the central rib 14, which in turn is generally keel or fin-shaped,can slide.

Respective screws, indicated 18 and 19, extend through holes in the endwall of the punch 1 (that is, through the face of the punch 1 whichfaces away from the mouth of the cavity 17). The screws 18 and 19 extendrespectively into the central rib 14 and into each intermediate side rib15 so as to regulate the sliding of the ribs into the cavity 17 againstthe reaction force exerted by respective springs 18a, 19a which arefitted around the shanks of the screws 18 and 19 within the cavity 17.Each spring acts between the end wall of the cavity 17 and the rib (thecentral rib 14 or an intermediate side rib 15) through which thecorresponding screw 18 or 19 extends, urging the respective rib 14 or 15outwardly of the die 1.

The spring 18a associated with the central rib 14 usually has a lowerelastic constant than the spring 19a associated with the intermediateside ribs 15.

Thus, the arrangement is such that, starting from the rest positionshown in continuous outline in FIG. 12, the rib 14, and subsequently theribs 15, can be made to enter the cavity 17 gradually. This takes placewith a movement which, for the central rib 14, is represented by anintermediate position shown in chain line and an end position shown inbroken outline, the intermediate side ribs 15 travelling from the restposition indicated in continuous outline towards the end position shownin broken outline.

It will be appreciated that, when the ribs 14 and 15 are in their endpositions, the punch 1 also assumes a generally flat configurationcomplementary to that of the die 2.

As stated, the sheet F is pleated by interposing the generally flatsheet F between the punch 1 and the die 2.

At the start of the pleating operation (with the punch 1 and the die 2separated), the punch 1 is thus in the rest condition shown incontinuous outline in FIG. 12. As a result of the gradual movement ofthe punch 1 towards the die 2, the sheet F will then be engaged first bythe central rib 14 which will engage the corresponding central groove ofthe die 2, thus forming a first central pleat in the sheet F. As thecoupling movement of the two die elements 1 and 2 continues, the centralrib 14 will start to be retracted into the punch 1, so that theintermediate side ribs 15 start to engage the corresponding intermediateside grooves in the die 2, thus forming two further pleats in the sheetF which is being pleated.

Finally, as the coupling movement of the punch 1 continues furthertowards the die 2, the ribs 15 will start to move towards the end wallof the cavity 17 (like the central ribs 14, which continue to beretracted into the punch 1) so that the outer side ribs 16 come intoengagement with the corresponding grooves in the die 2, thus forming theoutermost pleats in the sheet F.

This sequence of steps is shown schematically in FIGS. 13 to 15 whichshow that the pleating of the sheet F is not achieved by a single blowbut, on the contrary, takes place gradually by the formation first ofthe central pleat, then of two side pleats on opposite sides thereofand, finally, of two further, outer pleats.

The sheet F is thus formed by a gradual movement during which the sheetF which is being pleated is drawn gradually towards the centre of thepleating device. This takes place under the yielding restraining actionof the strips 12 and 13.

As stated, the presence of a lacquer or resin which has a certainlubricating effect (for example, a polyethylene resin or lacquer) on theopposite faces of the sheet F causes the pleating described to takeplace gradually without jerks, thus avoiding any problems of tearing ofthe sheet F.

Naturally, the principle of the invention remaining the same, thedetails of construction may be varied widely from those described andillustrated, without thereby departing from the scope of the presentinvention.

I claim:
 1. A method of wrapping an article having at least one rounded portion, comprising the steps of:(a) providing a first sheet of material on the order of tens of microns thick and having a coating thereon which can perform a self-lubricating function; (b) pleating said first sheet of material gradually, progressively, and symmetrically from a central region thereof towards outer regions thereof, thereby forming symmetrical meridional pleats in a pleated portion of said first sheet of material, said pleating step including maintaining said first sheet of material generally flat and causing limited movement of said first sheet of material from said outer regions thereof toward said central region thereof while said pleats are being formed, said pleating step further including subjecting said first sheet of material to restraining action which opposes said limited movement of said first sheet of material from its outer regions toward its central region; (c) forming the pleated portion of said first sheet of material into a dished shape while substantially eliminating said pleats from said pleated portion by unfolding-type expansion of said pleats, thereby yielding a substantially smooth, wrinkle-free surface; (d) inserting an article into said dished shape and then joining a second sheet of material to said first sheet of material along a closed line such that said article is completely surrounded by said first and second sheets of material; and (e) cutting said first and second sheets of material along said closed line, thereby forming a rim which projects outwardly from said closed line.
 2. A method for forming wrappers for food articles, the method comprising the steps of:providing a sheet of metallic foil; forming pleats in an area of the sheet; and forming said area into a dished shape such that the pleats are virtually entirely eliminated in the area of the sheet forming the dished shape by unfolding-type expansion of the pleats, thereby yielding a generally smooth, substantially wrinkle-free surface.
 3. The method according to claim 2, wherein the step of forming pleats includes substantially symmetrically forming the pleats.
 4. The method according to claim 2, further including the step of maintaining the sheet generally flat during the step of forming pleats.
 5. The method according to claim 2, further including the step of causing limited movement of the sheet from outer regions of said area towards a center of said area during said step of forming pleats.
 6. The method according to claim 5, further including the step of subjecting the sheet to a restraining force which opposes said limited movement.
 7. The method according to claim 2, wherein the step of forming pleats includes forming the pleats progressively from a central region of said area towards outer regions of said area.
 8. The method according to claim 2, wherein the step of forming pleats includes forming the pleats gradually and progressively from a central region of said area towards outer regions of said area.
 9. The method according to claim 2, wherein the step of providing a sheet comprises providing a sheet with at least one face having a coating which can perform a self-lubricating function.
 10. The method according to claim 9, wherein the step of providing a sheet comprises providing a sheet with the coating on both faces of the sheet.
 11. The method according to claim 9, wherein the coating is a lacquer.
 12. The method according to claim 11, wherein the lacquer is a polyethylene-based lacquer.
 13. The method according to claim 2, wherein the metallic foil is aluminum.
 14. The method according to claim 2, wherein the sheet is less than one millimeter thick.
 15. The method according to claim 14, wherein the sheet is on the order of tens of microns thick.
 16. The method according to claim 15, wherein the sheet is about 10 microns thick.
 17. The method according to claim 13, wherein the step of providing a sheet comprises providing a sheet with at least one face having a coating which can perform a self-lubricating function and which sheet is less than one millimeter thick.
 18. The method according to claim 2, wherein the step of forming pleats comprises pressing the sheet between two complementary die elements.
 19. The method according to claim 2, wherein the step of forming said area into a dished shape comprises pressing the area between a punch and a die.
 20. The method according to claim 19, wherein at least one of the punch and the die has a vent hole and said method further comprises preventing the formation of aeriform masses trapped behind the sheet by discharging air through said vent hole.
 21. The method according to claim 2, wherein the step of forming pleats comprises forming meridional pleats.
 22. The method according to claim 2, wherein the step of forming pleats comprises maintaining said sheet generally flat and forming symmetrical, meridional pleats.
 23. The method according to claim 2, wherein the step of forming pleats comprises forming the pleats gradually and progressively from a central region of said area towards outer regions thereof while causing limited movement of said sheet from said outer regions toward said central region and while simultaneously subjecting said sheet to a restraining force which opposes said limited movement.
 24. A method for forming a wrapper for an article, the method comprising the steps of:providing a sheet of material having a thickness on the order of tens of microns; forming a predetermined area of the sheet of material into a dished shape; and forming pleats in the sheet of material only in the predetermined area, prior to formation of the dished shape, the pleats being of a size such that they are virtually entirely eliminated during formation of the dished shape by unfolding-type expansion of the pleats, thereby yielding a generally smooth, substantially wrinkle-free surface.
 25. A device for forming pleats in a portion of sheet material which subsequently is to be formed into a dished shape, the device comprising:a male die element which has ribs for forming the pleats in the portion of sheet material, the ribs being arranged along meridians of a first pleating area corresponding to the portion of sheet material that is to be formed into the dished shape; a female die element which has grooves that are oriented along meridians of a second pleating area and which are complementary to the ribs; the ribs and the grooves of the male and female die elements being only within the first and second pleating areas, respectively; and wherein of the set of ribs and the set of grooves, one set is in a generally fixed position on its respective die element and the other set is mounted so that it can be retracted relative to its respective die element during mating of the die elements, as the pleats are formed, in a sequential manner such that pleating is carried out in distinct portions of the sheet material sequentially; and resilient means for opposing retraction of said other set.
 26. The device according to claim 25, wherein the ribs and the grooves are arranged in a substantially symmetrical configuration.
 27. The device according to claim 25, wherein the ribs and the grooves have generally V-shaped cross sections.
 28. The device according to claim 25, wherein the ribs and the grooves have generally V-shaped cross sections and are arranged in a substantially symmetrical configuration.
 29. The device according to claim 25, wherein at least one of the male die element and the female die element has restraining means in the region surrounding its respective pleating area for exerting a restraining force on the sheet material which opposes movement of the sheet material towards the centers of the first and second pleating areas.
 30. The device according to claim 29, wherein the restraining means can yield resiliently.
 31. The device according to claim 29, wherein the restraining means is constituted by strips of foamed material.
 32. The device according to claim 29, wherein there are restraining means on both the male and female die elements.
 33. The device according to claim 32, wherein the restraining means comprise a substantially continuous covering on one of the die elements and discontinuous formations on the other die element.
 34. The device according to claim 25, wherein the elastic constants of the resilient means are greater for the ribs or grooves situated toward outer portions of the respective pleating area than for the ribs or grooves situated toward inner portions of the respective pleating area. 